Optically-transparent biological samples, such as isolated biological cells and certain microorganisms in culture, are three-dimensional dynamic entities, constantly changing their biophysical features, shapes, volumes, and spatial locations. These intrinsic transients can occur over time scales ranging from days to less than milliseconds. Tracking these dynamic phenomena can provide valuable spatial-temporal data for cell biology studies, as well as provide early indications of malfunctions due to disease which may be used as biomarkers. Wide-field digital interferometry (WFDI) is a label-free technique providing quantitative measurements of the optical path delays (OPDs) associated with optically-transparent samples. By recording the interference pattern between the light or beams that have interacted with the sample and the mutually-coherent reference wave, WFDI may capture the complex wavefront of sample field, containing information on the three-dimensional structure of the sample.
WFDI phase measurements of transparent biological sample dynamics are frequently performed by use of modified Mach-Zehnder or Michelson interferometric setups, where at least one microscope objective is inserted in the beam path to magnify the sample. The reference and sample arms in these conventional interferometers have different optical paths. The introduction of instability in the interferometric system, including differential vibrations or air perturbations in the interferometer arms, can cause measurement errors, and may significantly degrade the quality of the recorded phase profile of the sample.
Common-path interferometers can provide a partial solution to the aforementioned instability problem. This can be achieved by creating an overlap between the reference and sample beams, so that the same vibrations occur for both beam paths. Common-path interferometers can be useful in ambient conditions, where the interferometric system is not constructed on vibration-isolating optical tables, or the system cannot be enclosed to avoid differential air perturbations in the two interferometer arms.
Common-path interferometry can be implemented using on-axis WFDI geometry, requiring the acquisition of several phase-shifted interferograms for each instance of the sample, or using off-axis geometry, requiring the acquisition of only one interferogram per sample instance. The latter approach, however, requires the use of a diffraction grating and often other elements in the beam paths, which necessitates a dedicated optical system design, and an additional alignment process.
Accordingly, in view of the foregoing, there is a need for improved interferometric systems and techniques for measuring biological samples or other samples.